1. Field of the Invention
This invention relates to a dome-shaped flexible contact structure formed in a sheet metal structure, and more particularly to flexible means for providing electrical contact between adjacent conductive structures shielding radio frequency interference.
2. Background Information
Increased operating speeds, i.e. clock frequencies, in computer systems continually result in increased tendencies of various circuits within such systems to generate electromagnetic energy at radio frequencies. Part of such energy may be emitted from a system to form radio frequency interference (RFI). Because energy of this kind can interfere with communication signals external to a computer system, various governments and agencies place strict limits on the levels of RFI which can be emitted from a system.
Various changes in component technology and design have resulted in electronic circuits which are both faster in operation and lower in electrical power requirements. Such circuits are typically also more sensitive to electromagnetic interference from various external sources.
A traditional method, both for reducing the level of RFI emitted by circuits within a system and for reducing the level of electromagnetic interference allowed to reach such circuits from external sources, is the placement of electrically conductive shielding material around the circuits. This method is proven to be quite effective in accomplishing these objectives when the shielding material forms an effectively continuous envelope around the circuits. However, since physical access to electronic circuits of this kind is often required for upgrading, repairing, or otherwise adjusting the circuits, various means are typically employed to allow the separation of portions of shielding material into two or more structures, with electrical contacts being used to provide conductivity along seams between the separable structures.
The spacing between adjacent contact points along a seam is typically determined in response to the various fundamental and harmonic frequencies of electromagnetic energy generated by the circuits enclosed by the shielding material. As system operating speeds, i.e. clock frequencies, increase, the associated higher frequencies of electromagnetic energy require a decreased spacing between adjacent contact points.
An important example of this kind of shielding is provided in a typical personal computer having a conductive shielding enclosure formed by a metal frame and a by a metal liner within a removable top cover. The use of the metal liner allows the structural and externally visible portions of the top cover to be made from a thermoplastic resin, which can easily provide the structural and aesthetic properties needed in such an application. The top cover must typically be easily removable by the system user to replace various internal components, such as circuit cards, or to upgrade the system by adding new components within spaces provided for this purpose. The main portion of the top cover generally includes sides extending downward to the frame at right and left ends, so that the removal of the cover optimally exposes components within the computer for access. To form an effective shield, the conductive liner within the top cover must be electrically connected to the metal frame along the various seams where the top cover lies adjacent to the frame.
Electrical connections between the top cover and the frame must be easily separable as the cover is removed by the user. These connections must also be reliably re-established as the cover is replaced on the frame. Since adjacent connections must be spaced close together along the seams between the frame and top cover, a large number of such connections are required for an individual cover. It is thus desirable that the contact force established at each such connection should be relatively light to allow the removal and reattachment of the cover without undue effort.
Since variations in the dimensions of the frame and cover, as produced by typical manufacturing processes, may result in variations of several millimeters in the spacing between adjacent features of the frame and top cover, it is further desirable that a system of electrical contacts between the frame and top cover should be flexible enough to provide appropriate contact forces over a wide range of displacement distances.
Furthermore, in many personal computer systems, the combination of frame and top cover is designed so that the top cover is removed as it is pushed toward the rear and then lifted upward. Various structures holding the cover down are cleared by this rearward motion before the cover can be removed upward. The re-installation of the cover follows the reverse of this procedure, with the cover being pushed downward before it is slid forward. Thus, it is particularly desirable that a contact system for providing electrical conductivity between the frame and top cover be configured so that sliding in two directions, perpendicular to each other but parallel to the seam between the frame and top cover, should be accommodated without compromising the structural properties of the contact system.
In the establishment of electrical contact to prevent the emission of RFI, wiping contact is particularly desirable to remove various types of contamination from both surfaces in the contact area as adjacent parts are brought together. The establishment of a normal force without wiping may leave contamination between adjacent contact surfaces, preventing the formation of an effective electrical contact.
In the application of establishing contact between the frame and top cover of an electronics enclosure, as described above, the relative motion between the top cover and frame assembly, occurring as the top cover is installed, is often easily adequate to clean the interface between contact surfaces of the top cover and frame. However, if this interface is formed using springs attached to one side or the other, i.e. to the frame or top cover, in such a way that repeated wiping is not established between the springs and the surface to which they are attached, the electrical contacts between the springs and the surfaces to which they are attached may not be as reliable as the electrical contacts formed at the interface between the springs and the surface on which they slide.
It is therefore desirable that a spring contact system should either be formed as an integral part of a conductive shield, or that a spring contact system should alternately be formed in such a way that relative motion occurs at the interface between the two sections (i.e. the frame and top cover), and also along the surface of attachment between the spring contacts and the shield to which they are attached.